1. Field of the Invention
The present invention relates to a pneumatic tire in which heat-generation can be controlled and tire wear resistance can be improved simultaneously. More particularly, the present invention relates to a pneumatic tire suitable for off-the-road, for example vehicle for construction in which deterioration in tire wear resistance is minimized and tire life can be prolonged by suppressing generation of heat.
2. Description of the Related Art
Conventionally, in order to suppress generation of heat in a tread of a pneumatic tire, physical properties of a rubber material of a tread rubber layer have been controlled.
However, because deterioration in tire wear resistance generally arises in tread rubbers which have physical properties controlled so as to suppress generation of heat, it was difficult to suppress generation of heat without causing any deterioration of tire wear resistance.
In view of the aforementioned facts, an object of the present invention is to provide a pneumatic tire in which generation of heat can be suppressed without causing any deterioration of tire wear resistance.
In order to attain the above object, a first aspect of the present invention is a pneumatic tire which comprises: a tread portion constituted with a tread rubber layer and including grooves formed at a radially outer surface thereof i.e., a tread surface of a tire; and a thin tread reinforcing layer provided at a middle portion, in the thickness direction (xe2x80x9cthe thickness directionxe2x80x9d indicates the direction normal to the tread surface as the ground-contacting portion hereinafter), of the tread rubber layer so as to extend over at least one portion of the tread rubber layer in the tire widthwise direction thereof, the tread reinforcing layer being composed of a material (such as rubber) having larger tensile modulus of elasticity than that of a rubber composing the tread rubber layer. Note that the aforementioned expression of xe2x80x9cthe middle portionxe2x80x9d indicates the middle portion of a gauge of the tread rubber layer in the thickness direction.
A pneumatic tire according to the first aspect achieves the following effects.
FIG. 14A is a schematic view of a conventional tread rubber layer. The tread rubber layer 100, when contacting the ground, undergoes compressive deformation in the vertical direction thereof due to force F, and a middle portion of the tread rubber layer 100 in the vertical direction thereof expands in the lateral direction.
When a tire rotates during running, the tread rubber layer 100 repeatedly undergoes the above-described deformation and generates heat.
As shown in FIG. 14B, the pneumatic tire according to the present aspect includes a tread reinforcing layer 102 (for example, the tread reinforcing rubber layer) which is composed of a material (e.g., rubber) having larger tensile modulus of elasticity, i.e., being less stretchable than a rubber composing the tread rubber layer 100, at the middle portion of the tread rubber 100 in the vertical (thickness) direction thereof. The tread reinforcing layer 102 suppresses the lateral expansion of the tread rubber layer 100. As a result, the compressive deformation of the tread rubber layer 100 is also suppressed.
As the amount of lateral expansion and the amount of vertical compressive deformation is decreased, generation of heat in the tread rubber layer can be suppressed.
According to the present aspect, providing the tread reinforcing layer (the aforementioned tread reinforcing rubber layer, for example) can effectively suppress generation of heat in the tread rubber layer. In the case of a tread structure which includes the tread reinforcing rubber layer as described above, in addition to suppression of heat-generation, tire wear resistance can be ensured by choosing the rubber having physical property for tire wear resistance of the tread rubber layer. Thus, suppression of heat-generation is readily achieved without causing any deterioration of tire wear resistance.
In a second aspect of the present invention, in the pneumatic tire having the first aspect, a thickness of the tread reinforcing layer is in a range of from 2 to 5% of a thickness of the tread rubber layer.
A pneumatic tire according to the second aspect achieves the following effects.
When the thickness of the tread reinforcing layer is less than 2% of the thickness of the tread rubber layer, the capacity of the tread reinforcing layer to suppress the lateral expansion of the tread rubber layer is lowered.
On the other hand, when the thickness of the tread reinforcing layer exceeds 5% of the thickness of the tread rubber layer, the influence of the wear resistance of the tread reinforcing layer on the wear property of the entire tread rubber layer will increase to such an extent that it cannot be ignored. In addition, the influence of the heat generated by the tread reinforcing layer on the heat-generation property of the entire tread rubber layer will increase to such an extent that it cannot be ignored.
Accordingly, it is preferable to set the thickness of the tread reinforcing layer to the range of from 2 to 5% of the thickness of the tread rubber layer.
In a third aspect of the present invention, in the pneumatic tire having the first and second aspects and including the tread reinforcing rubber layer, tensile stress at 50% elongation of the rubber composing the tread reinforcing rubber layer is in a range of from 2 to 4 times as large as the tensile stress at 50% elongation of the rubber composing the tread rubber layer.
A pneumatic tire according to the third aspect achieves the following effects.
When the tensile stress at 50% elongation of the rubber composing the tread reinforcing rubber layer is less than 2 times as large as the tensile stress at 50% elongation of the rubber composing the tread rubber layer, the capacity of the tread reinforcing rubber layer to suppress the lateral expansion of the tread rubber layer is lowered.
On the other hand, when the tensile stress at 50% elongation of the rubber composing the tread reinforcing rubber layer exceeds 5 times as large as the tensile stress at 50% elongation of the rubber composing the tread rubber layer, there is a possibility that the rubber layers of different types (physical properties) separate from each other.
Accordingly, it is preferable to set the tensile stress at 50% elongation of the rubber composing each tread reinforcing rubber layer to the range of from 2 to 4 times as large as the tensile stress at 50% elongation of the rubber composing the tread rubber layer.
In a case in which a plurality of tread rubber layers are used, the tensile stress at 50% elongation of each tread reinforcing rubber layer must be set to the range of from 2 to 4 times as large as a tensile stress at 50% elongation of an adjacent rubber composing the tread rubber layer.
In a fourth aspect of the present invention, in the pneumatic tire having any one of above aspects, the tread rubber layer is made up of a plurality of rubber layers stacked in the tire radial direction, and each of the plurality of rubber layers has different physical properties.
A pneumatic tire according to the fourth aspect achieves the following effects.
If only one type of rubber is used, there may be cases where properties which are incompatible to each other cannot be improved simultaneously. In the pneumatic tire according to the present aspect, however, tire wear resistance and suppression of heat-generation can be further improved, simultaneously, with a structure in which, for example, an outer layer in the tread rubber layer at a ground-contact surface side thereof is composed of an NR-based or a SBR-based cap rubber which improves tire wear resistance, and an inner layer in the tread rubber layer near a carcass side thereof is composed of an NR-based base rubber which suppresses generation of heat.
In a fifth aspect of the present invention, in the pneumatic tire having any one of above aspects, a position depth that is a distance from the tread surface to the outer side of the tread reinforcing layer in the thickness direction when the tire is new is in a range of 10 to 30% of a depth of a groove when the tire is new.
A pneumatic tire according to the fifth aspect achieves the following effects.
Suppression of heat-generation in the tread rubber layer is necessary for a new or a slightly worn tire in which the tread rubber layer is thick (i.e., the tread rubber layer makes relatively large movements and is hard to release heat). Accordingly, it suffices that the effect of the tread reinforcing rubber layer is achieved only in a period when the tire is new or slightly worn. After this period, the effect of the tread reinforcing layer may be diminished or lost.
According to the present aspect, by setting the a position depth that is a distance from the tread surface to the outer side of the tread reinforcing layer in the thickness direction when the tire is new to a range of from 10 to 30% of the depth of the groove of a new tire, the effect of the tread reinforcing layer to suppress the heat-generation is achieved in an optimal state in the period when the tire is new or slightly worn during which suppression of heat-generation is most often required.
In a sixth aspect of the present invention, in the pneumatic tire having any one of above aspects, the material (e.g., rubber) composing the tread reinforcing layer has different color (specifically, color, brightness and saturation) than that of the rubber composing adjacent tread rubber layer, and the a position depth that is a distance from the tread surface to the outer side of the tread reinforcing layer in the thickness direction when the tire is new is set to 30% of the depth of the groove of a new tire.
A pneumatic tire according to the sixth aspect achieves the following effects.
When the material (e.g., rubber) composing the tread reinforcing layer has different color (color, brightness and saturation) than that of the rubber composing an adjacent tread rubber layer disposed at the radially outer side of the tread reinforcing layer, it can visually and thus easily be recognized that the tread reinforcing layer appears at the tread surface as a result of wear because the color (color, brightness and saturation) of the tread surface changes as the tire is used.
As black rubber is commonly used for tires, if the same black color is used for the tread reinforcing layer, it is preferable to alter the color (color, brightness and saturation) of the tread reinforcing layer from that of the adjacent tread rubber. Alternatively, it is preferable to employ a different color (other than black) for the component material of the tread reinforcing layer.
In the case of pneumatic tires for a construction vehicle, under a normal usage condition, front and rear tires are removed and re-mounted at different positions at the time when the tread rubber layer is worn by 30% (the tread rubber layer is supposed to be xe2x80x9cwornxe2x80x9d by 100% when the groove disappears).
Accordingly, by setting the a position depth that is a distance from the tread surface to the outer side of the tread reinforcing layer in the thickness direction when the tire is new to 30% of the depth of the groove of a new tire, it can visually and easily be recognized that the tire is worn by 30% when the tread reinforcing layer of different color (color, brightness and saturation) appears at the tread surface.
In a seventh aspect of the present invention, in the pneumatic tire having any one of above aspects, short fibers are added in the tread reinforcing layer.
A pneumatic tire according to the seventh aspect achieves the following effects.
Because the short fibers are added in the tread reinforcing layer, the tread reinforcing layer can be made less stretchable than the rubber composing the tread rubber layer.
In addition, according to the present aspect, when the tread reinforcing layer is provided as the tread reinforcing rubber layer, the tread reinforcing rubber layer can be prepared by adding short fibers in the rubber of the same type as that composing the tread rubber layer. As a result, a common rubber can be used for both the tread rubber layer and the tread reinforcing rubber layer.
In an eighth aspect of the present invention, in the pneumatic tire having the seventh aspect, the short fibers are oriented in a plane which is substantially parallel to the surface of the tread reinforcing layer.
A pneumatic tire according to the eighth aspect achieves the following effects.
When the short fibers are added in the material (e.g., rubber) composing the tread reinforcing layer, the tread reinforcing layer is made less stretchable when the short fibers are oriented parallel to the surface of the tread reinforcing layer than when they are oriented parallel to the thickness direction of the tread reinforcing layer.
Accordingly, it is preferable that the short fibers are oriented in a plane which is substantially parallel to the surface of the tread reinforcing layer.
By continuously extruding, from a slit-like extrusion port of an extruder, an unvulcanized rubber in which the short fibers are added, the short fibers can be oriented in a plane which is substantially parallel to the surface of the tread reinforcing layer.
In a ninth aspect of the present invention, in the pneumatic tire having any one of the first, second, fourth, and fifth aspects, a rubber-fiber composite structure including fibers having larger tensile modulus of elasticity than that of the rubber composing the tread rubber layer is provided as the tread reinforcing layer, in place of the tread reinforcing rubber layer.
A pneumatic tire according to the ninth aspect achieves the following effects.
In the pneumatic tire according to the present aspect, the rubber-fiber composite structure which includes fibers having larger tensile modulus of elasticity than that of the rubber composing the tread rubber layer and is less stretchable than the tread rubber layer is provided in a middle portion, in the thickness direction, of the tread rubber layer. Because the rubber-fiber composite structure suppresses lateral expansion of the tread rubber layer when the tread rubber layer contacts ground, the amount of compressive deformation of the tread rubber layer can be made small.
As the rubber-fiber composite structure diminishes the amount of lateral expansion and the amount of vertical compressive deformation of the tread rubber layer, generation of heat in the tread rubber layer can be suppressed.
In short, according to the present aspect, generation of heat in the tread rubber layer can be effectively suppressed by providing the rubber-fiber composite structure. In addition, tire wear resistance can be ensured by choosing the rubber having physical property for tire wear resistance of the tread rubber layer. Therefore, suppression of heat-generation is readily achieved without causing any deterioration of tire wear resistance.
Further, according to the present aspect, because the direction in which the tread rubber layer is less stretchable can be controlled or adjusted by the orientation of the fibers in the rubber-fiber composite structure, it is possible to set the orientation of the fibers to a direction in which the suppression of heat-generation in the tread rubber layer can be most effectively achieved.
In a tenth aspect of the present invention, in the pneumatic tire having the ninth aspect, the rubber-fiber composite structure is disposed substantially parallel to the tread surface.
According to the tenth aspect, by disposing the rubber-fiber composite structure substantially parallel to the tread surface, the effect of suppressing the lateral expansion of the tread rubber layer can further be improved.